The present invention relates generally to the gamma correction of the viewing angle of a liquid crystal display, and more particularly, to a circuit which utilizes a plurality of digital-to-analog converters to adjust the viewing angle of a liquid crystal display. Compared to other display devices, liquid crystal displays operate at a low voltage in a limited amount of space, and they are light weight and low cost. As a result, liquid crystal displays are widely used as displays for computers, calculators, and security systems. In addition, liquid crystal displays have been particularly useful in avionic instrumentation display units.
The majority of modem military, commercial, and private aircraft instrumentation panels utilize ARINC cutouts for the avionic instrumentation display units. In other words, there is a standard instrumentation panel cutout for each avionic instrumentation display unit. As a result, only a limited amount of space is available to insert and wire the avionic instrumentation display units. At the same time, however, a need exists for additional processing devices in the avionic instrumentation display units. Consequently, space-conservative liquid crystal displays have largely replaced bulky cathode ray tube displays in avionic instrumentation display units.
Liquid crystal displays produce images by manipulating the orientation of liquid crystalline substances. The birefringence of a layer of liquid crystalline substance can be adjusted by applying a voltage across the layer. Consequently, the light which is transmitted through a layer of liquid crystalline substance can be controlled by changing the voltage across the layer.
In a typical liquid crystal display, segmented electrodes are utilized to generate the images of the display. The segmented electrodes are arranged in a pattern on the display, and they are driven individually by electronic circuitry. By applying voltages to a desired combination of segmented electrodes, the electronic circuitry can control the amount of light which is transmitted through the segmented electrodes. In this manner, the electronic circuitry can manipulate the image which is produced on the liquid crystal display.
However, it is well known to display engineers that liquid crystal displays which utilize the aforementioned technology suffer from a narrow field of view. The viewing quality of a liquid crystal display is partly determined by its contrast ratio. The contrast ratio of a liquid crystal display is limited by the amount of light that leaks through the segmented electrodes while they are in a dark state. Typically, the maximum contrast ratio of a liquid crystal display is achieved only within a narrow viewing angle centered about normal incidence. As the viewing angle is increased, the contrast ratio is diminished due to increased light leaking through segmented electrodes which are in dark states. Consequently, it becomes increasingly difficult to see images on the display as the viewing angle is increased.
As a result of the narrow field of view, the effectiveness of liquid crystal displays has been compromised in applications that require wide viewing angles. In particular, the narrow field of view has limited the effectiveness of liquid crystal displays in avionic instrumentation display units. Avionic instrumentation display units may require a wider field of view such that a pilot and a co-pilot can simultaneously view the display. Therefore, a need exists to increase the field of view of liquid crystal displays.
In order to address this need, many liquid crystal displays are pivotable such that the user may rotate the display for maximum contrast. However, a pivotable display may create reliability problems, and it increases the cost and complexity of the display system. Moreover, a pivotable display does not increase the field of view. It merely changes the direction of the display screen. Therefore, a pivotable display is not viable for applications that require a wide field of view or multiple users.
As mentioned above, it is also known that a bias voltage can be applied across a layer of liquid crystalline substance in order to control the amount of light that is transmitted through the layer. A bias voltage can be used in this manner to adjust the viewing angle of a liquid crystal display. In order to adjust the bias voltage, it is known to use a potentiometer or a combination of fixed and variable resistors. The accuracy and stability of a potentiometer, however, decreases with wear. Moreover, the setting of a potentiometer can accidentally be moved if the adjustment knob is not recessed, and a screwdriver is required to change the setting of the potentiometer if the adjustment knob is recessed. On the other hand, a combination of fixed and variable resistors is commonly used in applications such as avionic instrumentation display units. However, the resistors in this type of circuit may have to be physically replaced in order to change the viewing angle of the display. Consequently, this method of adjusting the viewing angle is often not feasible or practical for applications such as avionic instrumentation display units that may frequently require an adjustment of the viewing angle.
In light of the shortcomings of known liquid crystal displays, a need exists for a method and apparatus for adjusting the viewing angle of a liquid crystal display without the use of resistor trees. Still another need exists for a programmable method and apparatus which is adapted to provide the desired gamma correction for any video signal source.
The present invention meets some or all of these needs by replacing known resistor trees with a plurality of digital-to-analog converters. The plurality of digital-to-analog converters are arranged in pairs which have a fixed relationship. By determining appropriate values for the digital-to-analog converters, a desired viewing angle may be obtained for a liquid crystal display.
In addition to the novel features and advantages mentioned above, other objects and advantages of the present invention will be readily apparent from the following descriptions of the drawings and preferred embodiments.